The tendency for molecules of any substance to spread out into the available space:

diffusion

neurons use exocytosis to release __________________

chemical signals

polar molecules and ions diffuse w/ the help of transport proteins that span the membrane:

facilitated diffusion

what are some functions of membrane proteins?

transporting solutes, pumping substances across a membrane, enzyme, signal transduction (signal causes change in protein that then relays a message to the inside of a cell), intercellular joining, cell-cell recognitions, attachemnt to the cytoskeleton and ECM.